The present invention relates to stents and, more particularly, to methods of fabricating and deploying stents.
The term “stent” has come into widespread use to denote any of a large variety of spring-like support structures, in the form of a tube which is open at both ends, which can be implanted inside a blood vessel or other tubular body conduit, to help keep the vessel or conduit open. Stents may be used following balloon angioplasty to prevent restenosis and may, more generally, be used in repairing any of a number of tubular body conduits, such as those in the vascular, biliary, genitourinary, gastrointestinal and respiratory systems, among others, which have narrowed, weakened, distorted, distended or otherwise deformed, typically as a result of any of a number of pathological conditions.
An effective stent must possess a number of important and very specific characteristics. Specifically, the stent should be chemically and biologically inert to its surroundings and should not react with, or otherwise stimulate, the living tissues around it. The stent must further be such that it will stay in the correct position and continue to support the tubular body conduit into which it is implanted over extended periods of time. Further, the stent must have the ability to return to its prescribed in-place diameter after the stent diameter has been significantly reduced prior to its insertion, usually tightly wrapped on a catheter, into the tubular body conduit.
These requirements limit the suitable metal stent materials to just a few metals and alloys. To date, it has been found that various alloys of nickel and titanium (hereinafter “nitinol”), with or without certain coatings, have the desired properties and are considered suitable for use in stent applications.
Specifically, nitinols, with or without special coatings, have been found to be chemically and biologically inert and to inhibit thrombus formation. Nitinols are, under certain conditions, also superelastic which allows them to withstand extensive deformation and still resume their original shape. Furthermore, nitinols possess shape memory, i.e., the metal “remembers” a specific shape fixed during a particular heat treatment and can resort to that shape under proper conditions. Shape-memory alloys can be formed into a predetermined shape at a suitable heat treatment temperature. At temperatures below the transition temperature range (“TTR”) certain nitinol alloys are in their martensite phase wherein they are highly ductile and may be plastically deformed into any of a number of other shapes. The alloy returns to its austenite phase, returning to its original predetermined shape upon reheating to a temperature above the transition temperature range. The transition temperature varies with each specific combination ratio of the components in the alloy.
The superelasticity of nitinols and their shape memory characteristics makes it possible to fabricate a stent having the desired shape and dimensions. Once formed, the stent can be temporarily deformed into a much narrower shape for insertion into the body. Once in place, the stent can be made to resume its desired shape and dimensions. Certain alloys of nickel and titanium can be made which are plastic at temperatures below about 30° C. and are elastic at body temperatures above 35°C. Such allovs are widely used for the production of stents for medical use since these nitinols are able to resume their desired shape at normal body temperature without the need to artificially heat the stent.
While such stents have been proven effective, they continue to suffer from a number of disadvantages. First, there is, in certain cases, a tendency for tissue to grow in the gads between adjoining loops of the stent. Over time, such growth could lead to the constriction, or even the complete closure, of the tubular body conduit in which the stent was introduced in order to keep open. A continuous, gap-free, tube structure with no gaps would eliminate such undesired tissue growth. However, a rigid tube would lack the highly desirable flexibility which a coiled spring configuration offers.
Another disadvantage is that the techniques for locating stents in a body conduit are such that the stents are often installed at a location which is not precisely the intended optimal location.
There is thus a widely recognized need for, and it would be highly advantageous to have, a stent which would be suitably flexible but which would significantly reduce, or even eliminate, the possibility of undesired tissue growth between the coils of the stent.
There is further a widely recognized need for, and it would also be highly advantageous to have, a technique for installing stents which would allow the stent to be located at precisely the desired location, either by controlling the stent design or by devising adequate methods for its accurate release. Furthermore, in those cases where the “shape memory” characteristic is used and the stent is to be heated in its final location in the body to cause it to resume its memorized shape, it is desired and advantageous to have a way of heating the stent which significantly reduces, or even eliminates, the chance of damaging surrounding tissue through heating which is conducted for too long and/or at temperatures which are too high.